1. Field of the Present Invention
The present invention relates to an exposure apparatus for use in the manufacture of integrated circuits and liquid crystal displays, and more particularly, to maintaining thermal stability of a holding stage on which a substrate is positioned.
2. Discussion of the Related Art
FIG. 6 shows a wafer stage 100 holding a photosensitive substrate such as a wafer 102. The wafer 102 is fastened in place on a wafer holder 104 by means of vacuum suction. The wafer holder 104 is supported on a wafer table 106, and the wafer table 106 is fastened to a base 108.
The wafer table 106 also includes a moving mirror 110 for measuring a position of the wafer stage 100 by means of an interferometer, a fiduciary mark 114 used in a base line checking sequence for measuring a positional relationship between an alignment system (not shown) and a projection optical system 112, and an irradiation monitor 118 for measuring a quantity of light passing through a reticle 116 and the projection optical system 112 in order to control magnification and focus of the projection optical system so as to maintain them at constant levels.
When exposure is initiated, energy of the exposing light is absorbed by a photosensitive material (such as a photoresist) coated onto a surface of the wafer 102. A chemical change occurs in the photosensitive material, and heat is generated in the process. As shown by arrows in FIG. 6, the heat is transferred from the wafer 102 to the wafer holder 104, and from the wafer holder 104 to the wafer table 106. As a result, the moving mirror 110 and the fiduciary mark 114 are heated. Also, if the energy of exposing light is measured by the irradiation monitor 118 prior to the initiation of exposure, heat is similarly transferred to the wafer table 106, so that temperatures of the moving mirror 110 and the fiduciary mark increase.
Furthermore, the wafer stage 100 is driven by a driving means (not shown), such as a DC motor or a linear motor, and heat is also generated when the motor is in operation. Accordingly, even movement of the wafer stage 100 raises the temperature of the wafer table 106, so that the temperatures of the moving mirror 110 and the fiduciary mark 114 increase.
It is important to measure the position of the wafer 102 accurately during exposure of the wafer 102. However, when the temperature of the wafer stage 100 increases for the reasons described above, relative positional relationship of the wafer 102, the moving mirror 110, and the fiduciary mark 114 may change, and the position of the wafer 102 cannot be measured accurately. Accordingly, it becomes impossible to achieve accurate alignment and positioning during the exposure.